feat(learn): 导航栏自学二级菜单，按语言记住学习进度

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/shared/composables/learnProgress.ts

@@ -0,0 +1,15 @@
+import { useStorage } from "@vueuse/core"
+import { STORAGE_KEY } from "utils/constants"
+
+export type TutorialType = "python" | "c"
+
+// 模块级单例：学习页写入、导航栏读取，必须共用同一个响应式引用，
+// 否则导航栏的链接不会随学习进度更新
+const learnStep = useStorage<Record<TutorialType, number>>(
+  STORAGE_KEY.LEARN_CURRENT_STEP,
+  { python: 1, c: 1 },
+)
+
+export function useLearnProgress() {
+  return { learnStep }
+}